Owing to the blooming development of various portable electronic products, the development of fabrication of elements focuses on microminiaturization and the flexibility of substrates. In view of the advantages of low cost and high flexibility, of low-temperature processes, in fabricating embedded elements or monolithic elements, processes for fabricating elements applied in flexible substrates are intended to focus on processes practicable in low temperature and printable processes.
The resistance of a negative temperature coefficient (NTC) thermistor decreases as a temperature increases. Since the temperature coefficient of resistance of the NTC thermistor is high and is stable in a wide range of temperatures, the NTC thermistor can be applied to temperature compensation, temperature measurement, liquid level sensing and current limit, and so on. For forming a spinel phase having semiconductor characteristics, the ceramic sintering temperature of sintered bodies is usually over 1200° C. On the other hand, thick film NTC thermistor ink is prepared by mixing ceramic powders having a negative temperature coefficient of resistance, a glass binder, and a polymer carrier. The thermistor is obtained by screen-printing the thermistor ink on insulating substrates and then performing sintering at a temperature above 700° C. However, the high fabrication temperature (above 700° C.) limits the application of such thick film NTC thermistor to ceramic substrates.
Moreover, WO9918581 and U.S. Pat. No. 5,980,785 are related to decreasing fabrication temperatures, wherein after low melting-point metals, such as Bi, Sn, Pb, In and Se, are mixed with high melting-point metals, the mixture is subjected to a thermal treatment by heating below 300° C. to form intermetallics showing the characteristics of a resistance or a NTC thermistor. Most of the low melting-point metals applied in the above method belong to the elements harmful to the environment, such as Pb, In and Sn. Furthermore, it is difficult to control the characteristics of the intermetallics, since the forming of the intermetallics is more temperature-sensitive. Besides, the B values of the intermetallics are usually below 2000. As compared with the high B values (e.g. 3000 to 4500) for the generally used NTC thermistor products, the intermetallics having lower B values cannot meet the criterion of being used in NTC thermistor.
To increase the efficiency of modulating the characteristics of elements, U.S. Pat. No. 7,135,955 discloses a method for fabricating a NTC thermistor by multi-layer ceramics processes. Two or more materials having different negative temperature coefficients are stacked on different layers. The purpose of controlling the eventual NTC thermistor characteristics is achieved by the arrangement of stacking different materials. There is no need to modulate the constitution and the amount ratio of the materials in response to different characteristics of elements. However, the element disclosed in this prior art still belongs to a ceramic element sintered at a high temperature. It tends to encounter diffusion and homogenization at interfaces between the stacked materials with different characteristics during high temperature co-firing, and even have reactions therewith taking place. Therefore, it is difficult to control the final characteristics of the fabricated element. Moreover, co-construction of different materials cannot surely satisfy the criteria of products both in B value and room temperature resistance value.
Accordingly, the problem to be solved here is to develop a method for fabricating a negative temperature coefficient thermistor in a low temperature, and the negative temperature coefficient thermistor can meet the demands for modulating both the B value and the room temperature resistance value.